This invention relates to a method of determining the diffusion coefficient of a diffusant in a solid particulate. In particular, it relates to passing an inert gas through a bed of particulates containing a diffusant, measuring over time a parameter proportional to the diffusant concentration in the effluent gas, determining the slope of the linear portion of that relationship, and multiplying that slope by a constant.
A diffusion coefficient indicates the rate at which a diffusant moves through a medium under a concentration gradient at a particular temperature and pressure. The diffusion of a chemical through a solid particulate is encountered in numerous industrial processes. Diffusion coefficients often must be known to properly design and operate these processes. For example, in a polyethylene manufacturing process, polymerization occurs in a flammable hydrocarbon solvent such as hexane. After the polymerization, the hexane solvent must be separated and recovered from the polymer to provide a clean resin product. The resin, usually in a form of powder, must be dried to a very low level to minimize the emission of hexane to the environment and the risk of explosion due to hexane build-up in storage vessels. When the hexane in the polyethylene is below 5%, the drying process becomes essentially a process of hexane diffusion in the polymer. The diffusion coefficient therefore is needed to properly design and optimize the process. As another example, crude poly(vinyl chloride) resins usually contain vinyl chloride monomer, a carcinogen. Its diffusion coefficient is needed to determine the conditions required to reduce the toxic vinyl chloride monomer concentration to a safe level.
Most conventional methods of measuring the diffusion coefficient in a plastic material are based on a film permeability method similar to ASTM D 1434. In such a method, a film made of the plastic material is placed between two chambers, one of which holds a constant concentration of the diffusant gas. The diffusant permeates through the film into the other chamber and, by measuring the diffusant concentration in the second chamber, one can obtain the diffusion coefficient. Although this method can produce good precision and accuracy for many practical applications, it has serious shortcomings if casting a film changes the morphology and physicochemical characteristics (such as crystallinity) of the material and if data for unaltered particulates are desired. In addition, at high temperatures and pressures, the mechanical integrity of the film may become a problem. And, for some materials, making a mechanically sustainable film simply is not possible.
Diffusion coefficients for large solids can be determined by measuring the gain or loss in weight of the solid over time as the diffusant enters or leaves the solid. The accuracy of this method becomes questionable when the diffusant concentration is low. Although the method can also be used to measure the diffusion coefficients for fine particulates, its accuracy falls due to interparticle contacts and interactions, and poor ventilation at those points.
Diffusion coefficients of polymer particulates can also be determined by, for example, packing a column with polymer-coated inert beads free of the diffusant, and chromatographing the diffusant by means of partitioning it between the polymer-coated beads and an inert carrier gas. The accuracy of that method also suffers because it requires coating inert particles, which changes the morphology of the polymer and may significantly affect diffusivity.